U.S. patent application number 12/192096 was filed with the patent office on 2009-02-26 for method and device for high-resolution imaging of test objects by electromagnetic waves, in particular for monitoring people for suspicious items.
Invention is credited to Frank Gumbmann, Michael Jeck, Lorenz-Peter Schmidt, Hue Phat Tran, Jochen Weinzierl.
Application Number | 20090051586 12/192096 |
Document ID | / |
Family ID | 37815358 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090051586 |
Kind Code |
A1 |
Weinzierl; Jochen ; et
al. |
February 26, 2009 |
METHOD AND DEVICE FOR HIGH-RESOLUTION IMAGING OF TEST OBJECTS BY
ELECTROMAGNETIC WAVES, IN PARTICULAR FOR MONITORING PEOPLE FOR
SUSPICIOUS ITEMS
Abstract
In order to image test objects by electromagnetic waves, in
particular millimetric waves, a test object is illuminated with the
electromagnetic waves, the scattered waves are received, and are
evaluated for a representation of the test object in the form of an
image based on the principle of "synthetic aperture radar" (SAR).
In order to allow as large an area as possible to be imaged with
high resolution in a short time, the phase centres of the
transmitting and receiving antennas are, according to the
invention, moved on a circular path parallel to the respective
digital focus planes of the imaging system, and are at the same
time shifted linearly in a further direction parallel to the
respective focus plane. The method can be used for monitoring
people for suspicious objects, for example for monitoring airline
passengers at an airport.
Inventors: |
Weinzierl; Jochen;
(Nuernberg, DE) ; Gumbmann; Frank;
(Oberreichenbach, DE) ; Tran; Hue Phat; (Erlangen,
DE) ; Schmidt; Lorenz-Peter; (Hessdorf, DE) ;
Jeck; Michael; (Mainz, DE) |
Correspondence
Address: |
Muncy, Geissler, Olds & Lowe, PLLC
P.O. BOX 1364
FAIRFAX
VA
22038-1364
US
|
Family ID: |
37815358 |
Appl. No.: |
12/192096 |
Filed: |
August 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2006/011630 |
Dec 5, 2006 |
|
|
|
12192096 |
|
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Current U.S.
Class: |
342/25A |
Current CPC
Class: |
G01S 13/9088 20190501;
G01V 8/005 20130101; G01S 13/887 20130101 |
Class at
Publication: |
342/25.A |
International
Class: |
G01S 13/90 20060101
G01S013/90 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2006 |
DE |
10 2006 006 962.5 |
Claims
1. A method for high-resolution imaging of a test object by
electromagnetic waves, in particular for inspecting individuals for
suspicious items, the method comprising: illuminating the test
object with electromagnetic waves; receiving the scattered waves;
and analyzing the scattered waves using a synthetic aperture
principle (SAR) to display an image of the test object, wherein
phase centers of a transmitting and a receiving antenna are moved
on a substantially circular path parallel to respective digital
focal planes of the imaging system and are substantially
simultaneously displaced linearly in another direction parallel to
the respective focal plane.
2. The method according to claim 1, wherein the transmitting and/or
receiving antennas are aperture radiators or horn antennas.
3. The method according to claim 1, wherein electromagnetic waves
with a frequency between 1 GHz and 10 THz, preferably between 30
GHz and 300 GHz, are used for illuminating the test objects.
4. The method according to claim 1, wherein a spatially invariant
correction filter is used in the SAR processing for reconstruction
of the raw data.
5. The method according to claim 1, wherein the raw data are
reconstructed using a spatially varying correction filter.
6. A device comprising: a transmitting and receiving antenna via
which electromagnetic waves are radiated and received; an analysis
system that uses the synthetic aperture radio (SAR) principle to
analyze received electromagnetic waves in order to obtain an image
of a test object; and an imaging system, wherein the transmitting
and receiving antenna are configured to be moved on a substantially
circular path parallel to respective digital focal planes of the
imaging system and are substantially simultaneously displaced
linearly in another direction parallel to the respective focal
plane.
7. The device according to claim 6, wherein the transmitting and
receiving antenna is rotatably supported on a rotary lever that is
rotatable by a rotary drive, and wherein the transmitting and
receiving antenna is connected to the rotary drive such that it has
substantially the same orientation for every position on the
circular path.
8. The device according to claim 6, wherein flexible lines in the
form of dielectric waveguides are used to conduct the transmit and
receive signals.
9. The device according to claim 6, wherein the electromagnetic
waves are millimeter waves.
10. The device according to claim 6, wherein the device is an
airport security screening device.
Description
[0001] This nonprovisional application is a continuation of
International Application No. PCT/EP2006/011630, which was filed on
Dec. 5, 2006, and which claims priority to German Patent
Application No. 10 2006 006 962.5, which was filed in Germany on
Feb. 14, 2006, and which are both herein incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] An aspect of the invention relates to a method for
high-resolution imaging of test objects by electromagnetic waves,
in particular for inspecting individuals for suspicious items, in
which the test object is illuminated with electromagnetic waves,
and the scattered waves are received and analyzed using the
synthetic aperture radar principle (SAR) to display an image of the
test object.
[0004] 2. Description of the Background Art
[0005] In order to inspect individuals or pieces of luggage
(hereinafter referred to as test objects) for hidden dangerous
items (weapons, explosives), imaging systems are known in which the
test objects (individuals, pieces of luggage) are scanned with
millimeter waves in order to detect suspicious items (U.S. Pat. No.
5,859,609). The advantage of these imaging systems is that
nonmetallic items as well as metallic items can be easily detected
due to their material-specific dielectric properties. Similarly,
the use of radar-based millimeter wave imaging systems in the field
of nondestructive testing of materials (NDT) has increased
sharply.
[0006] To ensure reliable detection of dangerous items when used in
the security field, or of defects in nondestructive testing of
materials, the imaging systems require high spatial resolution.
Furthermore, it is desirable, especially in applications in the
field of security, for the system to be capable of scanning the
largest possible area in a short period of time.
[0007] High resolution in one plane (lateral resolution) can be
achieved with focusing elements, for example elliptical mirrors or
dielectric lenses, which sharply focus the measurement signal on
the surface of the test object. However, this type of focusing has
inadequate depth of focus. High resolution is only provided in one
plane, the focal plane. A further disadvantage is that when
implementing fast scanning systems using large dielectric lenses or
mirrors to concentrate the measurement signals, rapid motion of the
large masses can be accomplished mechanically only with
difficulty.
[0008] German patent application 10 2005 042 463, which is
incorporated herein by reference, describes a method of the generic
type in which a test object is successively illuminated along its
circumference with millimeter waves, and the scattered millimeter
waves are received and analyzed using the synthetic aperture radar
principle (SAR) to display an image of the test object. A synthetic
aperture is created by the means that the waves emitted by an
antenna are first spatially concentrated, wherein the location of
high concentration is manipulated such that it serves as a moving
virtual antenna for SAR analysis.
[0009] In this imaging system using the SAR principle, it is known
to use no other focusing components for beam forming beyond one
antenna (monostatic) or multiple adjacent antennas
(quasimonostatic), for example horn antennas, for transmitting and
receiving radar signals reflected from the test object. The actual
focusing here is performed after the fact by digital signal
processing using the SAR principle. This type of data processing
permits digital focusing of the raw data for any desired distance
of the antenna from the test object. Additional focusing elements
such as mirrors or lenses can be omitted, thus drastically reducing
the mass of the millimeter wave sensors to be moved.
[0010] If a high lateral resolution in two dimensions (vertical and
horizontal) is to be achieved with the SAR processing, then the
phase center of each transmitting and/or receiving antenna must
also be moved in two dimensions. This is achieved in a known manner
by a linear motion in the horizontal and vertical direction. The
motion of two axes (a raster scan, for example) would be too slow
for a fast imaging system.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the invention to provide a
method for imaging test objects by means of electromagnetic waves
that can image as large an area as possible in a short time with
high resolution.
[0012] This object is achieved in accordance with at least an
embodiment of the invention in that the phase centers of the
transmitting and receiving antennas can be moved on a circular path
parallel to the respective digital focal planes of the imaging
system and are substantially simultaneously displaced linearly in
another direction parallel to the respective focal plane. The
linear displacement here can be in a vertical, horizontal, or
inclined direction.
[0013] As a result of the rotation of the antenna aperture, which
is to say the two-dimensional motion of the antenna phase center,
SAR processing of the signals is possible in the vertical and
horizontal directions.
[0014] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus, are
not limitive of the present invention, and wherein:
[0016] FIG. 1 shows a principle of the antenna motion,
[0017] FIGS. 2-5 each show parts of a mechanical structure in
various views,
[0018] FIG. 6 shows a representation of measured raw data in a
Cartesian coordinate system for signal processing,
[0019] FIG. 7 shows a flow diagram of the SAR processing with a
spatially invariant correction filter, and
[0020] FIG. 8 shows a flow diagram of SAR processing with a
spatially variable correction filter.
DETAILED DESCRIPTION
[0021] The imaging systems described below are each part of a test
unit used to inspect airline passengers at an airport. The test
unit is used to scan airline passengers for suspicious items such
as weapons or explosive substances during check-in. The
electromagnetic waves used for illuminating the test objects have a
frequency between 1 GHz and 10 THz. Preferably, millimeter waves
with a frequency between 30 GHz and 300 GHz are used. Either the
transmitting antennas themselves, or--as shown in FIGS. 3 through
5--antenna pairs with separate receiving antennas, can be used to
receive the reflected waves. Aperture radiators, in particular horn
antennas, are preferably used as transmitting and/or receiving
antennas.
[0022] The test unit can include a platform upon which the test
object, for example an airline passenger, is located while the
inspection is being carried out. In this process, the transmitting
and receiving systems rotate about the stationary test object in
order to successively illuminate it along its circumference with
millimeter waves. In addition, the test unit includes an analysis
system having suitable computing power, which uses the SAR
principle to analyze the received waves scattered by the test
object in order to obtain an image of the test object. The images
produced are displayed to an operator on suitable display
devices.
[0023] FIG. 1 illustrates the principle of the antenna motion.
Transmitting and receiving antennas (antenna pair 1) are movably
supported in the test unit such that their phase centers are moved
on a circular path parallel to the respective digital focal planes
2 of the imaging system and are simultaneously displaced linearly
in another direction parallel to the respective focal plane 2. For
purposes of simplification, the digital focal plane 2, which is to
say the surface to be scanned on the test object, is chosen to be a
plane parallel to the X-Y plane. The rotational motion of the
transmitting and receiving antennas 1 thus likewise takes place
parallel to the X-Y plane. The additional linear displacement of
the transmitting and receiving antennas 1 takes place parallel to
the respective focal plane 2, horizontally in the X direction
(arrow 3) in the example. The linear displacement can also take
place in a vertical or inclined direction. The test object is thus
scanned in a circle 4 that is displaced linearly. Since the focal
plane 2 is determined digitally, it can be chosen as a curved
surface, for example a cylindrical surface, or as a surface having
peaks and valleys.
[0024] The major parts of the mechanical structure of an inventive
imaging system are shown in FIGS. 2 (front view), 3 (rear view),
and 4 (side view).
[0025] It is necessary for SAR processing and for unambiguous
interpretation of the measurement results for the transmitting and
receiving antennas (antenna pair 1) to each receive the same
polarization direction during the rotation. A rotating polarization
is possible as well as a permanently fixed polarization direction.
FIGS. 2 through 4 show a fastening of the antenna pair in which the
polarization of the transmitted and received electromagnetic waves
is preserved during rotation of the antenna pair 1, and twisting of
the connecting lines is prevented.
[0026] To this end, the antenna pair 1 is rotatably supported in a
sleeve with a ball bearing. The sleeve is rigidly connected to a
rotary lever 5 that is supported in the device so as to be
rotatable about its center point by means of a rotary drive 6. In
addition, the antenna pair 1 is connected to the rotary drive 6 of
the rotary lever 5 through a combination of two gears 7, 8 and a
V-belt 9, wherein a transmission ratio of 1:1 is set, as shown in
FIGS. 3 and 4. The connection between the mount of the antenna pair
1 to the drive 6 of the rotary lever 5 through the gear/V-belt
transmission with a transmission ratio of 1:1 ensures that both
antennas of the antenna pair 1 have the same orientation for every
position on the circular path, as shown in FIG. 2. A counterweight
10 is fastened to the opposite end of the rotary lever 5 so that
the antenna pair 1 moves uniformly along the circular path of
radius r without imbalance.
[0027] The connection between the antennas and the transmitting and
receiving unit requires waveguides that have great flexibility,
even at high rotational speeds of the antennas. This would only be
achievable with enormous technical expenditure using coaxial or
hollow waveguide methods, especially for a frequency range above 50
GHz.
[0028] Consequently, as shown in FIG. 5, flexible lines in the form
of dielectric waveguides 11 with appropriate transition adapters to
hollow waveguide systems are used by preference. The dielectric
waveguides 11 are held in matched holders 12, 13, and conduct the
transmit signals 14 to the transmitting antenna, and conduct the
receive signals 15 from the receiving antenna to the analysis
system. Dielectric waveguides have the further advantage that they
have low attenuation in the millimeter wave region as compared to
hollow waveguide and coaxial line techniques.
[0029] The preferred signal processing is explained in detail below
with the aid of FIGS. 6 through 8.
[0030] In conventional SAR processing, a spatially invariant
correction filter is used. This means that the same correction term
is used for reconstructing every pixel. This allows processing with
conventional convolution or time-efficient FFT (fast Fourier
transform) algorithms.
[0031] This spatially invariant correction filter can be used in
the case of an imaging geometry with purely Cartesian, polar or
spherical coordinates. In the case of the rotating antenna pair 1,
which is additionally displaced linearly in a vertical or
horizontal direction, as shown in FIG. 1, a combination of linear
motion in Cartesian coordinates and in polar coordinates is
present.
[0032] In contrast to conventional SAR applications, this combined
motion requires a few additional processing steps. In an embodiment
two different methods of signal processing may be possible
according to the invention: an approximated SAR reconstruction in
Cartesian coordinates with spatially invariant correction term; or
an exact reconstruction in a mixed coordinate system (Cartesian and
polar coordinate system) with spatially varying correction
term.
[0033] The flow diagram of the SAR reconstruction in Cartesian
coordinates with spatially invariant correction filter (method 1)
is shown in FIG. 7.
[0034] In order to carry out SAR processing with conventional
algorithms from the literature, the measured raw data are first
mapped onto a Cartesian coordinate system. This mapping of the raw
data is shown in FIG. 6. In this process, the spacings of
individual grid points dx, dy in the new coordinate system are
chosen manually.
[0035] Due to the mapping of the rotational and linear motion into
a Cartesian coordinate system, a new raw data matrix with a
nonuniform pixel density results. In order to remedy this,
interpolation based on the principle of normalized averaging is
then carried out. This is followed by SAR processing with spatially
invariant correction filter. This can be carried out by
conventional SAR algorithms.
[0036] FIG. 8 shows a flow diagram of an exact reconstruction with
spatially varying correction filter (method 2).
[0037] The exact but more time-consuming reconstruction of the raw
data is carried out with a spatially varying correction filter in
this method. In this process, a separate correction term is
required for each pixel. The exact reconstruction of the test
object f(x, y, z=Z.sub.0) for a distance Z.sub.0 between the
transmitting and receiving antennas 1 and the test object can be
described mathematically as follows. Here, s(x, y, .omega.)
designates the raw data for various angular frequencies
.omega.:
##STR00001##
[0038] r.sub.0: Distance from coordinate origin to the pixel or
image point to be reconstructed
[0039] r.sub.p: Distance from coordinate origin to the surrounding
pixels or image points
[0040] .phi.: Angular position on circular path of
transmitting/receiving antennas
[0041] .DELTA.x: Step size of linear motion in horizontal or
vertical direction
f(x.sub.0,y.sub.0,z.sub.0,.omega.)=.intg..intg.s(x,y,.omega.)e.sup.j2k|
r.sup.p.sup.(x,y,=)-
r.sup.0.sup.(x.sup.0.sup.,y.sup.0.sup.,=.sup.0.sup.)|dxdy
{right arrow over (r)}.sub.p(x,y,z)=x{right arrow over
(e)}.sub.x+y{right arrow over (e)}.sub.y+z{right arrow over
(e)}.sub.=
{right arrow over (r)}.sub.0(x.sub.0,y.sub.0,z.sub.0)
=x.sub.0{right arrow over (e)}.sub.x+y.sub.0{right arrow over
(e)}.sub.y+z.sub.0{right arrow over (e)}.sub.=
[0042] The integration is replaced by a summation in the real case,
since only discrete measurement points are present.
[0043] The positions of the transmitting and receiving apertures in
Cartesian coordinates can be determined as follows: Coordinates of
the image point to be reconstructed:
x.sub.0={tilde over (x)}.sub.0+r cos (.phi..sub.0)
y.sub.0=r sin (.phi..sub.0)
z.sub.0=Z.sub.0
[0044] r: Radius of circular path of the transmitting/receiving
aperture
[0045] .phi..sub.0: Angular position on circular path of
transmitting/receiving antennas
[0046] {tilde over (x)}.sub.0: Position in horizontal or vertical
direction due to linear motion
[0047] Coordinates of the image points:
x={tilde over (x)}+r cos (.phi.)
y=r sin (.phi.)
z=0
{tilde over (x)}.epsilon.[{tilde over (x)}.sub.0-x.sub.3dB/2,{tilde
over (x)}.sub.0+x.sub.3dB/2],
.phi..epsilon.[.phi..sub.0+.phi..sub.3dB2,.phi..sub.0-.phi..sub.3dB/2]
[0048] r: Radius of circular path of the transmitting/receiving
aperture
[0049] .phi.: Angular position on circular path of
transmitting/receiving antennas
[0050] x.sub.3dB: Size of the antenna spot in the object plane with
reference to the x axis
x.sub.3dB=2Z.sub.0a tan (.phi..sub.3dB/2)
[0051] .phi..sub.3dB: 3db primary lobe width of the
transmitting/receiving aperture
[0052] In summary, the phase term
.phi.(x,x.sub.0,y,y.sub.0,z,z.sub.0) of the spatially varying
correction filter can be formulated as follows:
.PHI. ( x , x 0 , y , y 0 , z , z 0 ) = 2 k r .fwdarw. P ( x , y ,
z ) - r .fwdarw. 0 ( x 0 , y 0 , z 0 ) = = 2 k ( x - x 0 ) 2 + ( y
- y 0 ) 2 + ( z - z 0 ) 2 = = 2 k ( x ~ - x ~ 0 + r [ cos ( .PHI. )
- cos ( .PHI. 0 ) ] ) 2 + ( r [ sin ( .PHI. ) - sin ( .PHI. 0 ) ] )
2 + Z 0 2 ##EQU00001##
[0053] Following a pixel-by-pixel reconstruction with the spatially
varying correction filter, the data can again be mapped onto a
Cartesian coordinate system, and the effect of the nonuniform pixel
spacing can again be remedied by the normalized averaging
interpolation method. The invention being thus described, it will
be obvious that the same may be varied in many ways. Such
variations are not to be regarded as a departure from the spirit
and scope of the invention, and all such modifications as would be
obvious to one skilled in the art are to be included within the
scope of the following claims.
* * * * *